1. Field of the Invention
The present disclosure is directed at a method and system for desalinating saltwater using concentration difference energy. More particularly, the present disclosure is directed at a method and system for desalinating saltwater utilizing a dialytic stack designed to desalinate saltwater at a relatively high recovery ratio.
2. Description of Related Art
Over one quarter of Earth's population does not have adequate access to freshwater. Inadequate access to freshwater is detrimental, as it can lead to disease and malnutrition, limit agricultural development, and inhibit economic growth.
In contrast to freshwater, saltwater is readily available. Saltwater in the form of seawater constitutes about 97% of the water on Earth. Unless seawater is sufficiently desalinated, though, it is not only undrinkable but unsuitable for agriculture. “Desalination” refers to the process of removing anions and cations from saltwater. Seawater typically has a salt concentration of about 3.5% by mass; that is, about 35 grams of dissolved salt per liter of water. Another source of saltwater is salty, underground aquifer water, also known as “brackish water”. The salt concentration of brackish water typically ranges from less than 1% to more than 18% salt by mass. In contrast, drinkable water typically has a salt concentration of, at most, about 0.04%.
Several desalination methods are known in the art. One of the most popular methods at present is reverse osmosis (“RO”). RO involves mechanically forcing saltwater through spirally wound, semi-permeable membranes at high pressure. The membranes reject salt from the saltwater to produce both streams of freshwater and a salty, wastewater stream known as “brine”. Saltwater that is desalinated using RO requires extensive pre-treatment, however, which increases RO's energy requirements. RO also suffers from performance issues when the temperature of the saltwater is over about 30° C., which can be the case when the saltwater source is water from a warm ocean or power plant inlet, for example.
Another popular method of desalination is multiple effect distillation (“MED”) or multi-stage flash (“MSF”). MED and MSF desalinate saltwater by repeatedly evaporating and condensing the saltwater over a series of multiple stages. The source of the energy for MED and MSF processes is usually low pressure steam. The primary drawback of MED and MSF processes is the large amount of thermal energy they consume, which is typically an order of magnitude higher than that used by RO.
Another popular method of desalination is electrodialysis reversal (“EDR”), in which an electric current migrates dissolved salt ions through an electrodialysis stack composed of alternating anion and cation exchange membranes. One problem that occurs when desalinating saltwater using EDR and other methods that utilize ion exchange membranes is that the ion exchange membranes accumulate scaling over time, which inhibits desalination performance. Consequently, removing the scaling from the ion exchange membranes in an efficient manner is desirable.
Furthermore, all desalination processes produce brine in addition to freshwater. This brine typically has a salt mass concentration of about 7% for seawater desalination plants, and is of environmental concern as it is typically discharged back into a marine environment. Consequently, reducing one or both of the volume and concentration of brine from a saltwater plant would be beneficial.